1. Field of the Invention
This invention relates to laser scanning systems and laser pattern generators for precision laser direct imaging and mask making.
2. Description of Related Art
In photolithography, a raster scanning system is an optical device that scans one or more laser beams into a series of scan lines covering a photosensitive layer being patterned. A typical scanning system consists of one or more modulated lasers as sources of light beams, a mechanical or other device for scanning the beams through an angle, scan optics to convert the scanned beams into a set of scanned lines, and optics for matching the scanned lines to a desired image surface. Whether a raster scanner illuminates a specific region depends on the modulation of laser beam intensity as the beam scans past the region. A laser scanner's precision in selecting illuminated regions depends on the accuracy of modulation of the laser beam, the sharpness of the focus of the laser beam, the precision with which the laser beam moves across the layer being patterned, and synchronization between modulation and motion of the laser beam.
Laser direct imagers (LDIs) form a class of laser scanning systems that can replace reticle-based large area exposure devices. Typical LDI applications require exposure of ultraviolet sensitive photoresist, and require pixel sizes ranging from 25 to 2 .mu.m with pixel placement accuracy down to a tenth of a pixel. Scan optics for these precision imaging applications require that the optics concurrently provide diffraction-limited high resolution (low f/# or small spot size), along the scan line distortion correction, an image field suitable for accommodating multiple scan lines, and telecentric imaging. Additionally, for ultraviolet direct imaging, the system should have good radiometric efficiency. This requires that the system transmission efficiency be high. The power transfer can additionally be increased if the system can use more than one of the available UV spectral lines from a laser.
An additional requirement of the scan optics in a scanner is adequate working distance from the last optical element to the media for clearance of material handling equipment and accommodation of auxiliary optical devices. Typical auxiliary optical devices include light sensors used in synchronizing scan beam motion with scan beam intensity modulation and correction optics which reshapes or move the image of the scan line to account for movement or indexing of the device being patterned.
Related art can be found within existing UV mask making technology for semiconductor microlithography, conventional laser scanners used for printed circuit board pattern generators and for graphic art applications. For example, U.S. Pat. No. 4,796,038 describes a known pattern generator for reticle making. However, several limitations of such systems can be noted. First, systems such as described in U.S. Pat. No. 4,796,038 make reticles which does not require ultrafast write times. With a laser direct imager, time to image a substrate is of paramount importance since imaging time directly affects manufacturing cycle times. Thus, the LDI systems require high radiometric efficiency not provided in systems for making reticles. Critical system distinctions that demonstrate improvements to known systems are described below.